An image forming apparatus using an electrophotographic method, in which an electrostatic latent image is formed on a photoconductor (an image carrier), a toner being adhered on the latent image, and is transferred and fixed on paper and the like, is widely utilized. In particular, in an image forming apparatus such as an office printer and an on-demand printing apparatus for large volume printing which requires higher image quality and higher resolution, a wet type developing method utilizing a liquid type developer (also referred to as a wet type developer) which has a smaller toner particle size and hardly causes unevenness of a toner image has been utilized.
In recent years, has been proposed is an image forming apparatus utilizing a wet type developer with high viscosity and high concentration, which is constituted of solid toner including resin and pigment, which toner is dispersed in an insulating liquid “a carrier liquid (also referred to as a dispersing medium)” such as silicone oil at high concentration.
At the time of development by use of a wet type developer, it is general that development is conducted by forming a thin layer on the order of micrometer of a developer on a developer carrier such as a developing roller, and by bringing this developer having been made into a thin layer in contact with a photoconductor having a latent image.
A latent image on the surface of a photoconductor is developed by the thin layer of the wet type developer to form a toner image on the photoconductor surface. This toner image is transferred onto a recording medium. Otherwise, the toner image is once primarily transferred on an intermediate transferring material and then secondarily transferred onto a recording medium.
The toner image having been transferred onto a recording medium is fixed by being pressed or heated by a fixing device on a recording medium which is generally made of paper.
On the other hand, some amount of a wet type developer remains on the surface of the developing roller after development. Since this wet type developer (rich in a carrier component) provides bad influences to the following image formation, it is removed by a cleaning device such as a blade.
A wet type developer having been removed from the developing roller may be generally collected as waste liquid; however, it causes a high environmental load and needs high cost to dispose (including the case of recycling in other places after having been transferred or disposing after having been processed). Therefore, it may often be reused as a recovered developer.
In a development apparatus utilizing a wet type developer, in the case of recovering and reusing a wet type developer having been used in a developing section, the toner concentration of the recovered wet type developer is greatly deviated from the original toner concentration. If the developer is used as it is, it causes a problem of variation of image density.
Therefore, it is general to detect the toner concentration of the wet type developer having been recovered to adjust it.
However, it is difficult to use an optical sensor because of little transmitted light in the case of a wet type developer having high toner concentration while it is possible to directly measure the toner amount by an optical sensor in the case of a wet type developer having low toner concentration.
Therefore, for a wet type developer having high toner concentration, developed has been a technology to calculate toner concentration by measuring viscosity of the wet type developer as a substitute characteristic of the toner concentration (refer to Laid-open Japan Patent Application Publications No. 2008-309845 and No. 2009-2998). As a means to measure viscosity, utilized is a method to measure electric current of a motor required to stir the developer at a predetermined speed.
In Laid-open Japan Patent Application Publication No. 2008-309845, disclosed is a system to determine viscosity as a substitute characteristic of toner concentration by detecting a torque required to rotate a stirring means at a predetermined speed. Thus, toner concentration is detected by calculation based on the torque, temperature and mass of that liquid.
In Laid-open Japan Patent Application Publication No. 2009-2998, also disclosed is a system to determine viscosity as a substitute characteristic of toner concentration by detecting a torque required to rotate a stirring means at a predetermined speed. In that system, included is a technology to maintain a constant amount of a wet type developer by providing a detection tank with an opening through which the developer overflows when the amount becomes a predetermined value.
The viscosity of a wet type developer may be different even if the developer has the same toner concentration, depending on the temperature of the wet type developer and size and form of the toner.
With respect to the influence of temperature of a wet type developer on the viscosity, correction may be conducted by measuring the temperature (Laid-open Japan Patent Application Publication No. 2009-69586).
On the other hand, viscosity of a wet type developer may differ even if it has the same toner concentration when the particle size or form of the toner varies depending on manufacturing lots of the wet type developer. Therefore, there may be caused an issue of deviation of a toner concentration from an target value even if the viscosity of a wet type developer is adjusted to be constant.
Therefore, a mechanism to compensate the deviation has been proposed (refer to Laid-open Japan Patent Application Publication No. 2009-69586).
In Laid-open Japan Patent Application Publication No. 2009-69586, disclosed is a system to determine toner concentration by measuring viscosity. The basic correction table prepared for correction of a target viscosity characteristic depending on temperature is further corrected with respect to each developer to prepare an individual correction table for said developer. Then, adjustment of toner concentration is conducted based on the individual correction table.
As described above, viscosity of a wet type developer measured for adjustment of toner concentration may differ even with the same toner concentration due to influence of temperature of the wet type developer, and of variation in the particle size and form of a toner depending on manufacturing lots and the like.
To overcome this problem, developed was a technology to correct a target value of viscosity in consideration of temperature and manufacturing lots and the like, as described in Laid-open Japan Patent Application Publication No. 2009-69586.
However, in the case of practical toner concentration adjustment of a wet type developer, a wet type developer in use is supplied with another lot of wet type developer, whereby different lots of wet type developers get to be mixed together.
In a technology described in Laid-open Japan Patent Application Publication No. 2009-69586 correction is made only for each lot of wet type developer to set a suitable (viscosity) target value, and therefore that technology cannot be applied to the case of a wet type developer of a different lot being gradually added.
In addition to correction of individual lot, suitable correction of a (viscosity) target value depending on the supply amount of a concentrated developer of a different lot is required.